Controlled release pesticidal compositions offer several possible advantages over conventional compositions. First, they are usually more economical, as fewer pesticide applications to the crop are necessary. Controlled release compositions offer safety to the environment by preventing pesticide overuse and run-off or soil (translocation) leaching into unwanted neighboring areas such as water ways and wells. They can also offer safety to the crop in instances when large doses of conventional formulations are phytotoxic and provide safety to workers applying pesticides in the field by reducing the toxicity of the pesticide. Finally, controlled release compositions allow the effective use of pesticides which are too rapidly degraded, volatilized or leached away by rain in conventional formulations (i.e., conventional pesticides with very low residual activity).
The most common method of applying agricultural pesticides has involved addition to water as diluent and spraying. This generally requires that the dispersed particles be no greater in size in their largest dimension than 50 microns, preferable under 20 microns. Many processes for preparing controlled release products reported in the prior art would not provide such small particles suitable for spraying. These processes are summarized in Microcapsule Processing and Technology, A, Kondo (Marcel Denker, Inc., New York).
Major difficulties have been encountered in the production, storage and use of controlled release formulations depending on the active material employed. For example, it is especially difficult to encapsulate active materials that are solid and have low thermal and hydrolytic stability, such as carbamate insecticides and sulfonylurea herbicides. Essentially all of the prior art methods required have one or more disadvantages in that they (1) require heating, (2) require use of water or aqueous acid or base that can decompose hydrolytically unstable materials during the process or storage, (3) require a tedious step such as slow cooling or slow addition of a component, e.g., polymer precipitation agent, (4) require chemically active ingredients such as polyfunctional isocyanates or acid chlorides to form polymers, increasing the possibility of chemical attack on the active material, or contamination of the formulation with by-products of the polymerization reaction, and (5) do not produce particles small enough (under 50 microns in their longest dimension) to be conveniently sprayed without clogging spray nozzles.
Microencapsulation techniques have been reported that provide sprayable aqueous suspensions having slow release characteristics. A widely used process for microencapsulation involves interfacial polymerization, i.e., dispersion of a solution of active material in water by intensive mixing and then production of a polymeric shell at the interface of suspended microdroplets of active material and the continuous water phase. Interfacial polymerization requires the use of polyfunctional, chemically active monomers such as diacid chlorides of polyisocyanates in admixture with the active and a catalyst or complimentary monomer dissolved in the water phase. After the formation of the microcapsules, the aqueous microcapsule suspension can be treated with stabilizers and stored and later distributed as an aqueous suspension. A commercial example of this is Penn-Cap.RTM. M, a microencapsulated methyl parathion (Pennwalt Corporation). While this type of aqueous formulation is suitable for relatively stable liquids such as methyl parathion, it is not useful with bioactive materials that are either relatively insoluble in the organic monomers or show hydrolytic instability upon prolonged storage in the presence of water. Filtration of such small microcapsules and drying immediately after manufacture in an attempt to isolate them from the water for long-term storage is slow and adds two additional steps to the process.
The prior art also involves physical processes for preparing slow-release particles containing polymer barriers. The common feature of these is the use of preformed polymers.
U.S. Pat. No. 3,523,906 discloses the preparation of polymer microcapsules for agricultural or pharmaceutical use by dispersing an organic polymer solution containing the dissolved or dispersed active agent in water and precipitating the polymer by evaporation or organic solvent. This process employs a water-insoluble organic solvent for the polymer and generally produces particles over 50 microns in size that cannot be sprayed by conventional methods and solvent removal is a time consuming step that involves solvent recovery and recycling.
U.S. Pat. No. 3,523,907 discloses the preparation of polymer microcapsules by dispersing an organic polymer solution in water and adding an additional solvent that while miscible with water and the organic solvent, causes the precipitation of the polymer. Addition of the organic solvent to produce precipitation is a tedious and slow step requiring much care to avoid agglomeration of the particles.
U.S. Pat. No. 4,282,209 discloses the preparation of irregularly shaped controlled release particles (not microcapsules) by mixing an organic (acetone) solution of an active agent and a polymer with water (precipitating solvent) under high shear conditions. This process requires relatively large quantities of organic solvents that must be recycled. Also, physical abuse of the preformed particles during intensive mixing could produce cracks in the particles and consequently, too rapid a release rate.